In the field of manufacturing of ophthalmic lenses and the like, there is a great amount of interest in devising fabrication processes that are simple and economical, and which produce quality lenses requiring little or no finishing subsequent to manufacturing. Following these criteria, developmental efforts have focused on casting and molding of lenses from thermoplastic, thermosetting plastic, and polymerizable materials, due to the inherent economies of scale and the potential for reduced handling and labor costs of such processes.
In casting and molding processes in general, front surface and back surface dies are used to impart to the plastic material the optical surfaces appropriate for the desired ophthalmic correction. In casting processes in general, the dies are secured together by a gasket or the like at a desired spacing, and a liquid casting material is introduced in the casting cavity defined thereby, and allowed to cure and harden into a lens. A critical factor is the gasket, and that a large plurality of gaskets are required to accommodate and seal with the large plurality of differing optical surfaces of the various dies while also defining the desired die spacing (and lens thickness). Thus a large capital investment is required in gaskets and gasket tooling, as well as in the optical surface forming dies used in the casting process.
In molding processes in general, more rugged dies are employed to compress a moldable material in a closed molding cavity, impressing the optical surface configurations on the material as the material is hardened by thermal or chemical means. Generally speaking, an excess of molding material must be introduced into the cavity to allow for compressive die movement, and means must be provided to vent the excess material from the cavity and accommodate the sprues formed thereby. Also, as in the casting process, the dies must be urged together to the appropriate spacing to create the desired lens thickness. The amount of molding material, the venting of excess material, and the final thickness of the lens are interrelated factors that require precise process control to produce lenses of high quality for ophthalmic use.